a) Field of the Invention
This invention relates to a multimode focus detecting apparatus and, more particularly, to an active type multimode focus detecting apparatus in which a plurality of rays of infrared light is projected toward an object to be photographed and the light reflected from the object is received to thereby detect automatically an object distance.
b) Description of the Prior Art
An automatic focusing system which has been employed in cameras in the past may roughly fall into two categories: one is of a passive type where focus detection is performed on the basis of information of brightness distribution of an object to be photographed and the other is of an active type where beams of, for example, infrared light and ultrasound, are projected toward the object and focus detection is performed on the basis of reflection signals of the beams. Of these types, an automatic focus (hereinafter abbreviated to AF) device based on the so-called infrared active type triangulation focus detecting system can be realized with a simple arrangement and is employed in many products, which AF device is such that infrared light is projected toward the object through a projecting lens, the light reflected from the object is received by a semiconductor position detecting device through a light receiving lens disposed at a certain distance from the projecting lens, namely, at a distance of a base length, and an object distance is measured according to the position of incidence of the light.
Recently, the use of the active type AF device has accelerated the advance of the zoom and long focal distance of the camera and, in order to bring accurately the object to a focus even at tele-position, there is a need of making focus detection possible with respect to a farther distance.
Increasing a focus-detectable distance may be attained by one way of heightening the intensity of infrared light projected from the projecting lens and by the other way of increasing the S/N ratios of the semiconductor position detecting device and the processing circuit of a light receiving system. Of these ways, a packaged IRED (infrared emitting diode) is known, as a measure for increasing the projection intensity, from the past, in which by potting an optical resin on the light emitting surface of a light emitting element such as the IRED, the light emitting surface has a lens function to converge light diverging from the light emitting element so that rays of light are effectively introduced into the projecting lens disposed on the object side.
In the case of the active type AF device having a focus detecting zone only in the central area of its image plane, it has developed the defect that if a main object to be photographed does not lie in the direction of propagation of projected light, another object or a background will be brought to a focus, that is, a subject will come to a focus at infinity, with the result that the main object is in an out-of-focus condition on photographing.
Hence, this prior art device has required the operation of the so-called focus lock that the object to be photographed is previously set in a focus frame, disposed in a finder according to the picture composition, for indicating the direction of projection of infrared light, to perform focus detection, following which framing is reset for photographing. The technique popularly termed the wide-field AF or multi-point focus detection has thus proposed that the projecting signal for focus detection is used as a plural to determine a plurality of focus detecting positions in the finder, and thereby attempts have been made to eliminate the complicated operation such as in the focus lock.
However, when the packaged IRED is intended to realize such a multi-point focus detecting system, it has encountered the problems that the projection intensity, caused by a light emitting element, at the center of the visual field is increased, but the projection from another light emitting element, outside the center, makes convergent beams of light asymmetric because the light emitting element is not located on the optical axis of the packaged IRED and consequently, the form of projection intensity distribution is deteriorated as plotted in FIG. 1, which situation produces a great difference in focus detecting performance between the areas at the center and outside it.